HR: 0800h
AN: V51C-0585    [Abstracts]
TI: Ultra-high excess argon in kyanites - 15 Ga Ar/Ar age -
AU: * Itaya, T
EM: itaya@rins.ous.ac.jp
AF: Okayama University of Science, 1-1 Ridai-cho, Okayama, 700-0005 Japan
AU: Hyodo, H
EM: hhyodo@rins.ous.ac.jp
AF: Okayama University of Science, 1-1 Ridai-cho, Okayama, 700-0005 Japan
AU: Mikoshiba, M
EM: masumi-mikoshiba@aist.go.jp
AF: Geological Survey of Japan, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: Almino-silicate of Al2SiO5 has crystalographicaly three different phases of andalusite, sillimanite and kyanite. Kyanite as high pressure phase has been an important key to indicate high pressure condition of metamorphic rocks in comparison with the andalusite - sillimanite type of metamorphic sequence. The Abukuma mountains in northeastern Japan, a typical andalusite - sillimanite type (Miyashiro, 1958) has been under debate from a geological viewpoint as the kyanite - staulorite assemblage has been observed by Kano and Kuroda (1968) who proposed a polymetamorphism of the sequence. Hiroi et al. (1998) also found kyanite included in garnet, and described Cretaceous rapid loading and unloading of the sequence in a high temperature environment. Although the kyanite-bearing rocks are extremely rare in the sequence, kyanite and staurolite have been observed commonly in the river sand, suggesting a paleo Barrovian type of metamorphic terrain. The similar approach to find a Barrovian terrain has also been carried out in the Kitakami mountains by Uruno and Kitakami River Sand Recearch Group (1997) who found new occurrence of kyanite and staurolite in the river sand around the Tono granitic mass 200km north of the Abukuma mountains. Thus, the dating of kyanite may make a constraint for the Barrovian type metamorphism. Kyanite was concentrated from the river sand in the Kitakami mountains using the systematic acid treatment and heavy liquid technique, and finally was handpicked under microscope. They occur generally as a single crystal with clear and planer shape with cleavage and sometimes as a crystal aggregate of kyanite. Ar-Ar analyses of each kyanite crystal were carried out using laser fusion. The obtained ages are 7.7+-0.4, 9.9+-0.4, 11.1+-0.4, 15.1+-0.7 and 16.3+-1.5Ga which are two or three times older than the age of the earth. The ages older than the earth's have been reported from the diopsides from eclogites, ultramafic rocks and Zaire cubic diamonds (e.g., Dalrymple and Lanphere, 1969; Kaneoka, 1974; Ozima et al., 1989). These materials including kyanites are extremely low in potassium, and could be affected strongly by excess argon. The kyanites having 15Ga indicate that they occurred in the ultra-high excess argon environment of the crust in comparison with the mantle. This specific environment suggests that kyanite crystallizes under a ultra-high argon pressure in the old rocks, which come from some potassium bearing phases of low argon retentivity. The recent information for the ultra-high pressure rocks and the associated gneisses provide us a new concept on metamorphic terrain in the world that some of Barrovian type rocks were retrograded from the ultra-high pressure ones. The kyanite of 15Ga may be recrystalized under a specific environment with ultra-high argon pressure derived from radiogenic argon in phengites during the Barrovian type retrogression of UHP rocks.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting